A voltage controlled oscillator (VCO) or oscillator is a component that can be used to translate DC voltage into a radio frequency (RF) voltage or signal. In general, VCOs are designed to produce an oscillating signal at a particular frequency ‘f’ that corresponds to a given tuning voltage. In particular, the frequency of the oscillating signal is dependent upon the magnitude of a tuning voltage Vtune applied to a tuning diode network across a resonator circuit. The frequency ‘f’ may be varied from fmin to fmax and these limits are referred as the tuning range or bandwidth of the VCO. The tuning sensitivity of the VCO is defined as the change in frequency over the tuning voltage and it is desirable to tune the VCO over a wide frequency range within a small tuning voltage range.
The popularity of mobile telephones has renewed interest and generated more attention in wireless architectures. This popularity has further spawned renewed interest in the design of low noise wideband oscillators. The recent explosive growth in the new families of cellular telephones and base stations using universal mobile telephone systems (UMTS) has stirred a need for developing an ultra-low noise oscillator with a fairly wide tuning range. The demands of wideband sources have generally increased telescopically because of the explosive growth of wireless communications. In particular, modern communication systems are typically multi-band and multi-mode, therefore requiring a wideband low noise source that preferably allows simultaneous access to DCS 1800, PCS 1900 and WCDMA (wideband code division multiple access) networks by a single wideband VCO.
The magnitude of the output signal from a VCO depends on the design of the VCO circuit and the frequency of operation is in part determined by a resonator that provides an input signal. Clock generation and clock recovery circuits typically use VCOs within a phase locked loop (PLL) to either generate a clock from an external reference or from an incoming data stream. VCOs are often critical to the performance of PLLs. In turn, PLLs are generally considered essential components in communication networking as the generated clock signal is typically used to either transmit or recover the underlying service information so that the information can be used for its intended purpose. PLLs are particularly important in wireless networks as they enable communications equipment to lock-on to the carrier frequency onto which communications are transmitted relatively quickly.
The dynamic operating range and noise performance of a VCO may limit or affect the performance of the PLL itself, which in turn may affect the performance of the device in which the PLL is employed, e.g., RF transceivers, cell phone, modem card, etc. Broadband tunability of VCOs represents one of the more fundamental tradeoffs in the design of a VCO, impacting both the technology and the topology used. The dynamic time average quality factor (i.e., Q-factor) of the resonator as well as the tuning diode noise contribution affect the noise performance of a VCO. Furthermore, the dynamic loaded Q is, in general, inversely proportional to the operating frequency range of the VCO.
Despite the continuous improvement in VCO technology, low phase noise typically remains a bottleneck and poses a challenge to RF transceiver (transmitter—receiver) design. This is typically considered due to the more demanding parameters of the VCO design: low phase noise, low power consumption and a wide frequency tuning range.
In LC-resonator based VCOs, phase noise and power consumption typically depend primarily on the time average loaded Q-factor of the resonator circuit and the non-linearities associated with the tuning network, which typically employs varactors. The frequency tuning range is determined by the usable capacitive tuning ratio of the varactor and parasitic associated with the tuning network because the parasitic deteriorates and limits the effective tuning capability of the varactor at a high frequency. As the loss-resistance of the tuning network (e.g., varactor and resonator) determines the quality factor, attention is usually paid to the resistive behavior. The frequency range over which a coupled resonator circuit can be tuned by means of the tuning diode depends on the useful capacitance ratio of the tuning diode and on the parallel and series capacitance present in the circuit.
As the frequency for wireless communication shifts to higher and higher frequency bands, generation of an ultra-low noise, wideband, thermally stable and compact signal source at a relatively low cost becomes more and more challenging due to the frequency limitations of the active devices and broadband tunability of the tuning diode. In the past, wide tuning range and good phase noise performance were generally considered to be opposing requirements due to the problem of the controlling the loop parameters and the dynamic loaded Q of the resonator over the range of wideband operation.
For a varactor-tuned oscillator to be continuously tuned over a wide frequency range, the tuning diode should typically exhibit a large change in capacitance in response to a small change in the tuning voltage. However, this usually allows the tuning diode's own capacitance to be modulated by random electronic noise signals that are generated internally by various oscillator circuit elements, including the tuning diode itself. The tuning range of the oscillator generally influences the phase noise and typically there is a trade-off between the continuous tuning range of VCOs and the amount of phase noise generated by the varactor capacitance modulation. On the other hand, the requirements for low noise performance over the complete frequency range are typically demanding.
Thus, there exists a need for methods and circuitry for improving the phase noise performance over a wide tuning frequency range, typically more than an octave-band tuning range.